The invention relates to a cooling arrangement for an enclosed electric machine having an internal fan and an external fan lodged in a fan shell.
In a typical cooling arrangement disclosed in German Pat. No. 14 88 561 the stator housing is closed by two bearing brackets. Inside the machine, behind the drive-side bearing bracket, there is arranged on the shaft an internal fan which forces the internal air to circulate through the machine and the hollow fins. The machine is then cooled by the cooling air conveyed by an external fan disposed on the other shaft end, inside a fan shell. Such a cooling arrangement is not ideal for machines which are operated at variable speeds and possibly in changing directions of rotation, because the air transport depends on the rotational speed and because fans for both directions of rotation have a lower efficiency.
Published German patent application DE-OS No. 15 51 436 discloses an installable cooling arrangement where the internal and external fans are each driven by a separate motor, to achieve an external and internal air transport independent of speed and direction. There, however, the cooling arrangement is mounted as an assembly unit on the housing of the machine, covering the bearing bracket openings thereof. This expensive cooling arrangement however, requires considerable space and is usuable only for a horizontal shaft arrangement.
A compact cooling arrangement suitable for vertical or horizontal shaft arrangement of the machine and designed as an assembly unit with independently driven interal and external fans is known from German Pat. No. 835,476, where, however, both fans are driven by a common motor, the shaft of which or the motor housing traverses under seal the carrying end plate which closes off one end face of the stator housing and the two fans of which are mounted on the motor shaft on either side of the end plate. The internal fan runs between the end plate and a bearing bracket disposed inside the circumferentially closed stator housing which has axial cooling fins and which contains the openings necessary for the internal air circulation. Through openings adjacent to the shaft the internal air is forced into the interior of the machine again between the two bearing brackets through separate openings surrounding the former. As the two fans are driven at the same speed and as the heat transfer surface for the internal air is much smaller than would be available in the case of hollow fins, optimum cooling is not obtainable also because of the common fan drive, especially since an efficient air deflection surface in the internal air circulation is missing.